A variety of nonaqueous electrolyte secondary batteries have been used in recent years. For example, lithium ion secondary batteries, which are one kind of nonaqueous electrolyte secondary batteries have been widely used as secondary batteries of cell phones, notebook computers and so on owing to their compact size and large capacity. Lately, it also has been proposed to use nonaqueous electrolyte secondary batteries as batteries of electric vehicles, hybrid vehicles, and so on.
A nonaqueous electrolyte secondary battery has an active material capable of absorbing and releasing a charge carrier such as lithium (Li) at each of a positive electrode and a negative electrode. The nonaqueous electrolyte secondary battery works by moving the charge carrier between both the electrodes.
Carbon materials having multi-layered structures are mainly used as negative electrode active materials for nonaqueous electrolyte secondary batteries. Use of this kind of carbon material as a negative electrode active material can suppress a decrease in discharge capacity after repeated electric charge and discharge, and accordingly can improve cycle characteristics of a nonaqueous electrolyte secondary battery. However, such a nonaqueous electrolyte secondary battery having a negative electrode active material constituted only by this kind of carbon material has a problem of poor initial capacity (energy density).
In order to enhance the initial capacity of the nonaqueous electrolyte secondary battery, it has been proposed to employ an element which can be alloyed with a charge carrier and has a larger theoretical capacity than the carbon materials, as the negative electrode active material. Since silicon (Si) is an element which can be alloyed with a charge carrier such as Li and has a larger theoretical capacity than the carbon materials and other elements (e.g., tin and germanium), silicon is believed to be useful as the negative electrode active material for a nonaqueous electrolyte secondary battery. That is to say, a nonaqueous electrolyte secondary battery having a higher capacity can be obtained by using Si as the negative electrode active material than by using the carbon materials.
On the other hand, Si makes a great volume change associated with Li absorption and release in electric charge and discharge. This volume change causes Si to be changed into fine powder and drop or peel off from a current collector, so there arises a problem that such a battery has a short charge and discharge cycle life. This volume change associated with Li absorption and release in charge and discharge can be suppressed by using silicon oxide as the negative electrode active material instead of using elemental silicon as the negative electrode active material.
For example, use of silicon oxide (SiOx; about 0.5≦x≦1.5) as the negative electrode active material is being studied. SiOx is a generic chemical name of amorphous silicon oxide obtained from elemental silicon (Si) and silicon dioxide (SiO2) as raw materials. It is known that when subjected to heat treatment, SiOx decomposes into silicon (Si) and silicon dioxide (SiO2). This is called a disproportionation reaction and if homogeneous solid silicon monoxide (SiO) having a ratio of Si to O of approximately 1:1 is subjected to heat treatment, the silicon monoxide is separated into two phases of silicon (Si) phase and silicon dioxide (SiO2) phase by an internal reaction in the solid. The Si phase obtained by the separation is very fine and dispersed in the SiO2 phase. Moreover, the SiO2 phase covering the Si phase serves to suppress decomposition of an electrolytic solution. Therefore, a nonaqueous electrolyte secondary battery using a negative electrode active material comprising SiOx which has been decomposed into Si and SiO2 has good cycle characteristics.
By the way, SiOx has relatively poor electric conductivity. Accordingly, a negative electrode including SiOx as the negative electrode active material also has poor electric conductivity. Therefore, it is desired to improve electric conductivity of the negative electrode containing SiOx. As a method for improving electric conductivity of the negative electrode, it is considered to employ a negative electrode active material having good electric conductivity in combination with SiOx.
Graphite is known as a negative electrode active material having good electric conductivity. Graphite is generally used in a form of particles, and a coating layer such as a carbon coat is formed on a surface of graphite. Particles of this kind containing graphite are called carbonaceous particles. The carbonaceous particles of the present invention may have a coating layer and may be constituted only by graphite and have no coating layer.
PTL 1 mentions that a carbon material such as graphite can be suitably used as a negative electrode active material. PTL 1 describes that a carbon material for a negative electrode active material having a small particle diameter (e.g., an average particle diameter of 15 μm or less) are preferred. However, it has been difficult to attain both capacity enhancement of a battery and an improvement in electric conductivity of a negative electrode by simply selecting graphite having a small particle diameter.